1. Field of the Invention
The present invention relates to a light emitting device with a light emitting element composed of an anode, a cathode, and a film that contains an organic compound capable of emitting light upon application of electric field (the film is hereinafter referred to as organic compound layer), and to a method of manufacturing the light emitting device. Specifically, the present invention relates to a light emitting device using a light emitting element that is lower in drive voltage and longer in element lifetime than conventional ones. A light emitting device in this specification refers to an image display device that uses a light emitting element. Also, the following modules are all included in the definition of the light emitting device: a module obtained by attaching to a light emitting element a connector such as an anisotropic conductive film (FPC: flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package); a module in which a printed wiring board is provided at an end of a TAB tape or a TCP; and a module in which an IC (integrated circuit) is directly mounted on a light emitting element by the COG (chip on glass) method.
2. Description of the Related Art
Light emitting elements are drawing attention as the next-generation flat panel display elements for their characteristics including being thin and lightweight, fast response, and direct current low voltage driving. Also, being self-luminous and having wide viewing angle give the light emitting elements better visibility. Therefore the light emitting elements are considered as effective elements for display screens of electric appliances and are being actively developed.
It is said that light emitting elements emit light through the following mechanism: a voltage is applied between electrodes that sandwich an organic compound layer, electrons injected from the cathode and holes injected from the anode are re-combined at the luminescent center of the organic compound layer to form molecular excitons, and the molecular excitons return the base state while releasing energy to cause the light emitting element to emit light. Molecular excitons generated in organic compounds take either singlet excitation or triplet excitation. This specification deals with elements that emit light from singlet excitation and elements that emit light from triplet excitation both.
These light emitting elements are classified by driving methods into passive matrix (simple matrix) type and active matrix type. The ones that are attracting attention most are active matrix type elements, for they are capable of displaying images of high definition with the QVGA level number of pixels or more.
An active matrix light emitting device having a light emitting element has an element structure as the one shown in FIG. 2. A TFT 202 is formed on a substrate 201 and an interlayer insulating film 203 is formed on the TFT 202.
On the interlayer insulating film 203, an anode (pixel electrode) 205 is formed to be electrically connected to the TFT 202 through a wiring line 204. A material suitable for the anode 205 is a transparent conductive material having a large work function. An ITO (indium tin oxide) film, a tin oxide (SnO2) film, an alloy film of indium oxide and zinc oxide (ZnO), a semi-transparent gold film, a polyaniline film, etc. are proposed. Of those, the ITO film is used most because it has a band gap of about 3.75 eV and is highly transparent in the range of visible light.
An organic compound layer 206 is formed on the anode 205. In this specification, all the layers that are provided between an anode and a cathode together make an organic compound layer. Specifically, the organic compound layer 206 includes a light emitting layer, a hole injection layer, an electron injection layer, a hole transporting layer, an electron transporting layer, etc. A basic structure of a light emitting element is a laminate of an anode, a light emitting layer, and a cathode layered in this order. The basic structure can be modified into a laminate of an anode, a hole injection layer, a light emitting layer, and a cathode layered in this order, or a laminate of an anode, a hole injection layer, and a light emitting layer, an electron transporting layer, and a cathode layered in this order.
After the organic compound layer 206 is formed, a cathode 207 is formed to complete a light emitting element 209. The cathode is often formed of a metal having a small work function (typically, a metal belonging to Group 1 or 2 in the periodic table). In this specification, such metal (including alkaline metals and alkaline earth metals) is called an alkaline metal.
A bank 208 is formed from an organic resin material to cover the edges of the anode and prevent short circuit between the anode and the cathode at the site.
FIG. 2 shows one pixel and the light emitting element formed therein. The actual pixel portion is provided with a plurality of light emitting elements each structured as shown in FIG. 2 to constitute an active matrix light emitting device.
In the above-described conventional structure for a light emitting device, the interlayer insulating film and the anode (transparent conductive material) formed on the interlayer insulating film have different thermal expansion coefficients. When heat treatment is performed on a structure in which materials having different thermal expansion coefficients are in contact with each other as in this conventional light emitting device structure, it causes a crack in the interface on the side of the material that has the smaller thermal expansion coefficient (the anode, in this case). The anode is an electrode for injecting holes that participate in light emission into the organic compound layer. If there is a crack in the anode, the crack affects generation of holes, reduces the number of holes injected, and even degrades the light emitting element itself. The irregularities of the surface of the anode also affect generation and injection of holes.
Furthermore, the organic compound layer is by nature readily degraded by oxygen and moisture. Despite this fact, organic resin materials such as polyimide, polyamide, and acrylic are frequently used to form the interlayer insulating film and oxygen or other gas released from this interlayer insulating film degrades the light emitting element.
Moreover, the cathode of the light emitting element is formed of an alkaline metal material, such as Al or Mg, which can seriously impair TFT characteristics. An alkaline metal mixed in an active layer of a TFT causes a change in electric characteristic of the TFT, making it impossible to give the TFT a long-term reliability.
In order to avoid impairing TFT characteristics, it is preferable to prevent alkaline metal contamination of an active layer of a TFT by separating a TFT manufacture step processing room (clean room) from a light emitting element manufacture step processing room (clean room). However, another problem arises when moving a substrate between rooms (clean rooms) is added to the manufacture process in order to prevent the alkaline metal contamination; the TFT substrate may be contaminated by dusts or other contaminants in the air, and the TFT element may be damaged by electrostatic discharge.